Tilt rotor control

ABSTRACT

A system for driving a tilt rotor between vertical and horizontal using a variable displacement motor controlled in response to a swash angle of the motor measured in a feedback loop.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.18275270.9 filed Dec. 31, 2018, the entire contents of which isincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to tilt rotor control systems andparticularly drive assemblies for rotating pylons or hubs of tilt rotorsbetween their horizontal ‘airplane’ mode and their vertical ‘helicopter’mode.

BACKGROUND

Some aircraft are provided with tilt rotors which are pivotable suchthat the rotors make take a vertical or ‘helicopter’ position or ahorizontal or ‘airplane’ position. Such aircraft usually have a pair ofsuch rotor systems. In the horizontal position, the aircraft operateslike a conventional propeller-driven airplane. In the vertical position,the aircraft is able to hover. Here, the terms vertical and horizontalare used to describe the orientation of the rotor hub or pylon.Sometimes the terminology is reversed, with the terms horizontal andvertical describing the plane of rotation of the rotor blades. In thelatter case, the ‘horizontal’ mode is the helicopter or hover mode andthe vertical mode is the airplane mode.

A drive mechanism moves the rotor system between the horizontal andvertical positions. This is in many cases a hydraulic drive mechanism,but other drive mechanisms e.g. electric may also be used. The drivemechanism comprises a series of links operated by means of a motor.

During normal flight, the aircraft will operate with the rotor systemsin the horizontal or airplane position. For reasons of safety andreliability, it is important for the rotor system to be secured in placein the aircraft position sufficiently to resist counter-forces from theair (airload) and other vibratory forces which are usually high duringflight, otherwise, the rotor system could induce aircraft control andstructural strength issues

To ensure this, the rotor system hubs or pylons are pre-loaded intostops.

Challenges arise in the motor control of the drive mechanism due to thepre-load. Traditionally the motor is designed for high dynamicperformance against the maximum loads of both pylons during thetransitions from vertical to horizontal operation and vice versa. Thechallenge with preloading with the traditional systems is that the veryhigh stiffness end stops result in difficulty in achieving controlledend stop loads through this highly dynamic control.

There is, therefore, a need for a more efficient drive mechanism fortilt rotors.

SUMMARY

The present disclosure provides a drive mechanism as defined in claim 1;a tilt rotor system as defined in claim 3 and a method as defined inclaim 9.

Preferred embodiments will now be described by way of example only andwith reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a rotor system in the horizontal‘airplane’ mode.

FIG. 1B is a schematic view of a rotor system in the vertical‘helicopter’ mode.

FIG. 2 is a block diagram of the control system according to thedisclosure.

FIG. 3 is a graph showing an example load profile for conventionalsystems as the rotor retracts from the vertical to the horizontalposition.

FIG. 4 is a graph showing an example load profile for systems accordingto this disclosure as the rotor retracts from the vertical to thehorizontal position.

DETAILED DESCRIPTION

A tilt rotor system 10 generally comprises a rotor pylon 1 on which ismounted a hub 2 around which two or more rotor blades 3 are mounted. Theblades 3 are fixed to the hub 2 which rotates relative to the pylon 1during flight to provide a propulsive force or, in the helicopter mode,a lifting force, to move the aircraft. The rotor system is pivotablymounted to a part e.g. a wing (part of which is shown by 4) of theaircraft.

The rotor system is moved between the horizontal (FIG. 1A) and vertical(FIG. 1B) positions by means of a drive mechanism 5 including a seriesof links driven by a motor (not shown).

To secure the rotor system in the horizontal position (FIG. 1A) apre-load stop is provided comprising a spring 6 on one of the wing 4 andthe rotor system and a mating detent 7 on the other of the rotor systemand the wing.

In the vertical position as shown in FIG. 1B, a first linear actuator 5a of the drive mechanism is extended. To retract the rotor system to thehorizontal position, this actuator is driven by the motor to retract(here to telescope into the position shown in FIG. 1A) bringing thepylon 1 into the horizontal position. As the pylon approaches the endposition, the detent 7 will engage the end of the spring 6. Furtherretraction will cause the spring 7 to compress to its final securehorizontal position.

The motor power required to drive the actuator 5 a needs to besufficiently high to act against the increasing airloads acting againstthe rotor system as well as the spring force. The springs 6 are usuallyvery stiff.

As the motor size is usually designed to be as small as possible whilststill providing the required power, the motor will run at high speeds.This will result in very high inertia and kinetic energy.

Conventional systems use fixed displacement motors and so provide aconstant torque, while the speed is varied. As the pylon comes to thenear horizontal position, the loads acting on the pylon are considerablyincreased. As the pylon comes into contact with the end stop, forces canincrease by around 600%. Although force control loops are used tocontrol the load, such control loops suffer from high gain and highhysteresis of the actuation loads of the system. It has been found to bevery difficult to control, in particular, the pre-load part of the tiltmotion.

FIG. 3 shows how the load (airload and actuation (drag) load) varies fora conventional system as the pylon tilts from vertical to horizontal. Arelatively high load needs to be overcome initially to release the pylonfrom the vertical position (known as ‘breakout drag’). At the start ofthe actual pivot motion, the loads are relatively low but increase,initially gradually and then more steeply, as the pylon approacheshorizontal (running drag). At almost horizontal, the actuation loadincreases dramatically due to the end stop which can cause the motor tostall as it is operating too quickly for that load.

Also, as hinted at above, because the motor has to be designed tocontrol both airloads and internal forces and drags and also to alloweach system to provide back up in the event of failure of the motor ofthe other system, the motor is twice as big as it needs to be for mostof the operation, which is not efficient.

The system of the present disclosure uses, instead of the conventionalfixed displacement hydraulic motors, variable displacement motors whichallow for both variable speed and variable torque. The motor swashoperates in a speed control loop operating as an automatic load sensorand using the sensed load information to control the operation of thehydraulic actuator 5 a. This allows the tilt movement to be performed ina more controlled manner.

The speed control loop automatically ensures that the motor provides therequired capacity and, thus, torque for any part of the tilt operationso as to balance the loads in the system.

The control is provided by means of an intelligent algorithm thatmonitors the pylon's rotational position between vertical and horizontaland also monitors the drive motor's swash stroke as the system reachesthe horizontal position and where less power is needed, the swashincrease will be reduced.

The operation therefore automatically compensates for airload and anyprevalent actuation drag. The latter can vary considerably due totemperature.

The resulting loads over the range of pivotal motion are shown in FIG. 4where it can be seen, in particular, that the drastic rise in actuationload at the end stop is avoided, thus avoiding stalling of the motor.The motors do not, therefore, need to be designed large enough toprovide back-up for each other in the event of stalling.

The algorithm uses the knowledge that the swash of the motor duringoperation matches the motor shaft torque resulting from the airload andactuation drag. As the system approaches the end stop, the control loopwill monitor the change in swash as the speed is reducing due to theadded load, and limits the swash change to be within a given range e.g.10% of the operating load.

The operation of the system of this disclosure will be described in moredetail with reference to FIG. 2 .

The rotor system 10 is driven by a variable displacement hydraulic motor(VDHM) 20. This is part of a known power distribution unit PDU 30 whichwill not be described further as this will be well known to a personskilled in the art. Preferably, the VDHM is sized so that a single PDUcan drive both pylons of the aircraft. The motor will then be twice thesize needed for a single pylon.

A control signal is sent from a control system (not shown) from thecockpit or from the flight control system of the aircraft (not shown) toprovide a position demand 40 for the rotor system 10. This is forwardedto the VDHM 20 which, in turn, actuates pivoting of the rotor system 10to the desired position by driving actuator 5, 5 a.

Sensors detect motor speed 50, the tilt position 60 of the rotor systemand the motor swash angle 70. Conventionally, the system position 60 andmotor speed 50 would be used by speed control logic 80 to control thespeed of the tilt motion. In short, the position demand will be providedto the drive mechanism. Motor swash increases thereby increasing motoroutput torque which will accelerate the system. The system will pivotuntil the desired position is achieved. The speed will be limited orcontrolled by the speed control logic.

The system, in conventional systems, will be travelling at the pre-setspeed until the position sensor 60 indicates that the system has reacheda predetermined distance from the end stop, at which time the speed maybe reduced to avoid stalling. The kinetic energy of the system will beabsorbed by the end stop through strain energy. Even though the speedcontrol results in a reduced kinetic energy, the end stop is very stiffand this provides a high load to the system.

Using the algorithm 200 of the present disclosure the motor swash angleis determined and used to limit the change in swash to a predeterminedamount (e.g. 10%). The speed control loop provides a control signal tothe VDHM based on the position demand but adjusted for motor speed, thesystem position, the motor swash angle and the limit to change in swashangle in comparators 90, 100.

Because the algorithm monitors the swash and limits changes in swashwhich, in turn, limits the increase in system loading, the increase atthe end stop will be considerably less than in conventional systems.

The control system of the present disclosure, therefore, provides animproved control of movement of the rotor system taking the loads intoaccount automatically as they occur. The control system providescontinuous gauging of the system loads and drags.

The VDHM, by controlling motor swash and, thus, torque gain, can also beused to provide controlled torque during Built-In-Testing of the systemand in prognostics such as backlash measurements, measurements of thetorsional stiffness of the system. The VDHM can be used to apply precisetorque into the system in a static situation such as pre-loading asdescribed here.

While the present disclosure has been described with reference to anexemplary embodiment or embodiments, it will be understood by thoseskilled in the art that various changes may be made and equivalents maybe substituted for elements thereof without departing from the scope ofthe present disclosure. In addition, many modifications may be made toadapt a particular situation or material to the teachings of the presentdisclosure without departing from the essential scope thereof.Therefore, it is intended that the present disclosure not be limited tothe particular embodiment disclosed as the best mode contemplated forcarrying out this present disclosure, but that the present disclosurewill include all embodiments falling within the scope of the claims.

The invention claimed is:
 1. A tilt rotor system for an aircraft,comprising: a rotor pylon; an actuator connected to the rotor pylon tomove the rotor pylon between a first position and a second position,wherein the first position is a vertical position and the secondposition is a horizontal position; a drive mechanism to drive theactuator between the first and second positions, in response to an inputcommand; and a stop mechanism to secure the rotor pylon in the secondposition; the drive mechanism comprising: a variable displacement motorhaving sensors for measuring a swash angle of the motor, speed of themotor, and an actual tilt position of the rotor pylon, and means forproviding feedback control to the variable displacement motor to controlthe drive power of the motor based on the monitored swash angle, motorspeed and actual tilt position of the rotor pylon, and wherein the meansfor providing feedback control includes an algorithm configured tomonitor change in swash angle as the motor speed reduces and the actualtilt position of the rotor pylon approaches the second position and,based on the change in the swash angle, the actual tilt position of therotor pylon, and a reduced power requirement as the system reaches thesecond position, to limit change in swash angle to be within apredetermined range as a percentage of the operating load.
 2. A tiltrotor system as claimed in claim 1, wherein the stop mechanism includesa spring.
 3. A tilt rotor system as claimed in claim 1, furthercomprising a second rotor pylon and a second actuator connected to thesecond rotor pylon between a first position and a second position bymeans of the drive mechanism.
 4. An aircraft having a tilt rotor systemas claimed in claim 3, and input means for providing the input command.